1. Field of the Invention
The present invention is directed to a method employing nuclear magnetic resonance tomography for producing an image of fat plaque in an examination subject.
2. Description of the Prior Art
The presentation of calcification by computed tomography is usually employed as a non-invasive modality for diagnosing a constriction or a closure of arteries. This type of diagnosis is particularly important with respect to coronary arteries since a cardiac infarction can result from the constriction. The presentation of the calcification, however, is a relatively unreliable criterion for identifying the presence of constriction. First, the deposit of calcium need not necessarily indicate a constriction; second, a constriction can be present before a deposit of calcium occurs. The method hitherto applied is therefore unsatisfactory with respect to the sensitivity as well as with respect to the specificity.
The deposit of lipids (fats) in the vessels is considered a more reliable indicator for a constriction or a closure of arteries. Nuclear magnetic resonance tomography fundamentally allows fat deposits to be non-invasively displayed. A problem, however, is to display an adequate spatial resolution. Large coronary vessels have a diameter of approximately 3 through 5 mm. The fat plaque occupies 10 through 20% of the vessel diameter in an early phase and can amount to 70% later. An illustration of fat plaque in the coronary arteries and an evaluation of the degree of stenosis therefore presumes a spatial resolution in the sub-millimeter range. The spatial resolution also is degraded by the movement of the heart.
The presentation of coronary arteries in a MR image is known in general. In order to keep the measuring time optimally short, for example, the especially fast EPI (Echo Planar Imaging) technique is applied as proposed, among others, in the reference P. Mansfield, "Multiplanar Image Formation Using NMR Spin Echos", Journal of Physics C, 10 (1977). Since the originally proposed "single shot" EPI method wherein the k-space is sampled after one excitation makes extreme demands on the magnetic field gradients, a segmented EPI method is also often employed. After an excitation, only a part of the k-space is sampled, i.e. the entire measurement for the data of a tomogram comprises a number of excitations.
Methods are also known wherein essentially only fat is portrayed in the acquired image. These methods can be classified into methods having spectrally-selective excitation or saturation, and phase-difference methods. For example, the article by J. Pauly et al., Echo Planar Spin Echo and Inversion Pulses, MRM 29, pages 776-782 (1993), presents a possibility of designing excitation pulses which, under the influence of a gradient, are both spatially selective (i.e., for example, excite only one slice of an examination subject) and spectrally selective, so that, for example, only fat protons are excited. In the saturation method, the water protons are saturated in a preparation phase and the fat protons are subsequently excited, so that only the latter have a signal-producing effect.
The phase-difference method known, for example, from W. Thomas Dixon, Simple Proton Spectroscopic Imaging, Radiology 1984, 153, pages 189-194, makes use of the fact that the Larmor frequencies of fat and water protons are somewhat different, and thus the phase of the corresponding transverse magnetization diverges. By forming the difference at suitable points in time, the fat signal can be separated from the water signal.
The FFT (Fast Fourier Transform) method applied in a standard way in nuclear magnetic resonance tomography has the property that the entire field of view (FOV) is acquired with constant resolution per direction. U.S. Pat. No. 5,687,725 discloses a method wherein wavelet coding is used as an alternative to the FFT method. Individual image regions within an observation window can thereby be presented with higher resolution.
None of these methods supplies a satisfactory presentation of fat plaque.